The .cabal file allows you to specify a license file. Of course you can
use any license you like but people often pick common open source licenses
and it's useful if we can automatically recognise that (eg so we can display
it on the hackage web pages). So you can also specify the license itself in
the .cabal file from a short enumeration defined in this module. It
includes GPL, LGPL and BSD3 licenses.

This datatype indicates the license under which your package is
released. It is also wise to add your license to each source file
using the license-file field. The AllRightsReserved constructor
is not actually a license, but states that you are not giving
anyone else a license to use or distribute your work. The comments
below are general guidelines. Please read the licenses themselves
and consult a lawyer if you are unsure of your rights to release
the software.